Dieckmann also says that overfishing and the practice of throwing lower quality fish back into the sea to raise the value of fishing quotas might explain the massive drop in population.

"Human activity had a possibly irreversible evolutionary effect in just a few generations," he says.

"We are running up a Darwinian debt that future generations will have to pay back."

Some 15 years ago, cod stocks in the Canadian Grand Banks in the northwest Atlantic collapsed, bringing down the fishing industry in the region.

The same species is now under threat in the northeast Atlantic off Norway and Russia, he says.

In the Canadian Grand Banks fish stocks still show little sign of recovery, Dieckmann says, adding that evidence suggests humans are also responsible for this.

Looking at fishery data from the past few decades, the scientists found that increased mortality due to overfishing has favoured fish that mature smaller and earlier, yet also carry far fewer eggs at their first reproduction.

Older data shows that a typical cod caught in Norway might have taken 10 years to mature, while the same fish now would only take six years or even less, says Dieckmann.

"The question is not whether such evolution will occur, but how fast fishing practices bring about evolutionary changes and what the consequences will be," the scientists write, warning that such evolution may even be irreversible.

Dieckmann expects that a change coming about in 40 years might take up to 250 years to reverse, if it happens at all.

"Upsetting the dynamics of predators and prey may cause other changes that block this," he says.

Evolutionary impact

Assessing the evolutionary impact could become an essential tool in managing fish stocks, says Dieckmann.

Fishing policymakers could have helped avoid the collapse of cod stocks in the Atlantic by taking into account the fishing industry's impact on evolution in the oceans, and that might help prevent future catastrophes.

Dieckmann says recommendations for future fishing policy based on the research included: less fishing overall; avoiding catching small fish by using wider-meshed nets; and banning fishing in areas where fish spawn.

"Based on data that were available seven to 10 years before the collapse of the Grand Banks cod fisheries, an evolutionary impact assessment could have been used to send an early warning signal to policy makers," says Dieckmann.

"[Such assessments] applied now can thus help us avoid future catastrophes unfolding elsewhere," he adds.